1. Field of the Invention
The present invention relates to a high-strength and high-toughness sinter (ceramic composite material) and a process for producing the same. The high-strength and high-toughness sinter according to the present invention is used mainly for applications such as members of an internal combustion engine, e.g., a piston ring or an auxiliary combustion chamber, and members of a rocket engine, e.g., a nose cone or a nozzle.
2. Description of the Prior Art
Ceramics having excellent heat resistance known to the art include, e.g., oxide ceramics such as Al.sub.2 O.sub.3, B.sub.4 O, MgO, ZrO.sub.2, and SiO.sub.2, carbide ceramics such as SiC, TiC, WC, and B.sub.4 C, nitride ceramics such as Si.sub.3 N.sub.4, BN, and AlN, boride ceramics such as TiB.sub.2 and ZrB.sub.2, and silicide ceramics such as MoSi.sub.2, WSi.sub.2, and CrSi.sub.2. Molded articles of these ceramics have been hitherto prepared at a very high temperature. In recent years, a sintering assistant has been energetically studied for the purpose of lowering the sintering temperature and the sintering pressure. The sintering assistant serves to improve the sinterability of ceramics and, at the same time, to prevent the sinter particles from growing, so that not only the formation of voids among the particles is prevented but also the grain boundaries are packed at a high density.
Examples of the sintering assistant used in the art include MgO, NiO, CaO, TiO.sub.2, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, B.sub.4 C, B, and C. These additives are selected because they can bring about the occurrence of a phase reaction between the base ceramic and the additive so as to promote the sintering of the ceramic having a poor self-sinterability or because the sintering can easily proceed due to the formation of a plasticized liquid phase by the additive at a high temperature. Further, B and C can serve to enhance the sinterability through a lowering in the surface energy of SiC crystals.
However, when the above-described sintering assistants are present, there is a possibility that second and third phases are formed due to the reaction of a base ceramic with an assistant. These phases are present mainly at the crystal grain boundaries, and constituents of these phases bring about plastic deformation when exposed to a high temperature, which makes it impossible to produce a sinter having excellent high-temperature strength. For example, the addition of MgO to Si.sub.3 N.sub.4 brings about the formation of a vitreous phase comprising SiMgO.sub.3. Since this fills up the grain boundaries, an increase in the density can be attained. However, the mechanical strength of the sinter at a high temperature is sharply lowered at about 1,000.degree. C. due to the softening of the vitreous phase. In order to avoid the above-described lowering in the strength at a high temperature, it is preferred to select an assistant which does not form any vitreous phase. However, this kind of assistant is generally low in the ability of sintering, so that it becomes impossible to produce a satisfactory molded material.
As a means for eliminating the above-described inconvenience, a proposal has been made on a process for producing a ceramic sinter less susceptible to the lowering in the strength at a high temperature wherein a particular organometallic polymer is used as a binder for a ceramic powder and a mixture of the ceramic powder with the binder is heat-sintered.
For example, U.S. Pat. Nos. 4,336,215 and 4,556,526 each disclosed a process for producing a sinter which comprises heat-sintering a mixture of a polymetallocarbosilane with a ceramic powder after molding or simultaneously with the molding.
In the process described in the above-described U.S. Patents, the polymetallocarbosilane used as a binder of a ceramic powder is converted into an inorganic material when the mixture is heated at a high temperature. Since this inorganic material is a substance having a high melting point, the resultant sinter has relatively high strength even at a high temperature. This is because, as described on col. 6, lines 18 to 31 of the U.S. Pat. No. 4,336,215, the sinter produced in the process described in the above-described patents mainly comprises silicon carbide particles, a solid solution composed of SiC and TiC each produced by thermal decomposition of polytitanocarbosilane, and a grain boundary phase mainly composed of TiC.sub.1-x.
With respect to the strength of sinters produced by the processes described in the above-described patents, for example, a sinter having a deflective strength (bending strength) of 13.0 kg/mm.sup.2 was produced in Example 7 of the U.S. Pat. No. 4,336,215 by molding a mixture of a silicon carbide powder with polytitanocarbosilane and sintering the molded material at 1,200.degree. C., and a sinter having a deflective strength (bending strength) of 25.1 kg/mm.sup.2 was produced in Example 11 of the same U.S. Patent by preliminarily heating the above-described mixture at 600.degree. C., grinding the heated mixture, and hot-pressing the ground mixture at 1800.degree. C.
In recent years, engineering ceramics have been required to have higher functions. For example, the development of a sinter which has high strength and hardly brings about a lowering in the strength at a high temperature has been desired.